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                        <h1 id="creating-your-first-dll-in-rust"><a class="header" href="#creating-your-first-dll-in-rust">Creating your first DLL in Rust</a></h1>
<p>As a systems programming language with similar linkage support to that of C and C++, it is quite straightforward to build a DLL in Rust. Rust does however have it's own notion of libraries that are quite different to that of C and C++, so it's just a matter of finding the right configuration to produce the desired output.</p>
<p>As with most Rust projects, you can start with Cargo and get started with a basic template but it's so simple we'll just build it by hand here to see what's involved. Let's create a directory structure as follows:</p>
<pre><code>&gt; hello_world
  Cargo.toml
  &gt; src
    lib.rs
</code></pre>
<p>Just two directories and two files. There's the <code>hello_world</code> directory that contains the project as a whole. In that directory we have a <code>Cargo.toml</code> file that contains metadata for the project or package, information needed to compile the package:</p>
<pre><code class="language-toml">[package]
name = "hello_world"
edition = "2021"

[lib]
crate-type = ["cdylib"]
</code></pre>
<p>At a minimum, the <code>[package]</code> section includes the name and Rust edition your package is compiled with.</p>
<p>Rust-only libraries don't generally include a <code>[lib]</code>  section. This is necessary when you need to specifically control how the project will be used and linked. In this case, we're using 'cdylib' that represents a dynamic system library and maps to a DLL on Windows.</p>
<p>The <code>src</code> sub directory contains the <code>lib.rs</code> Rust source file where we can add any functions that we'd like to export from the DLL. Here's a simple example:</p>
<pre><code>#[no_mangle]
extern "system" fn HelloWorld() -&gt; i32 {
    123
}
</code></pre>
<p>The <code>[no_mangle]</code> attribute just tells the compiler to disable any name mangling and use the function name verbatim as the exported identifier. The <code>extern "system"</code> function qualifier indicates the ABI or calling convention expected for the function. The "system" string represents the system-specific calling convention which generally maps to "stdcall" on Windows.</p>
<p>And that's it! You can now build the package and it will produce a DLL:</p>
<pre><code>&gt; cargo build -p hello_world
</code></pre>
<p>Cargo will drop the resulting binaries in the target directory where you can then use them from any other programming language:</p>
<pre><code>&gt; dir /b target\debug\hello_world.*
hello_world.d
hello_world.dll
hello_world.dll.exp
hello_world.dll.lib
hello_world.pdb
</code></pre>
<p>Here's a simple example in C++:</p>
<pre><code class="language-C++">#include &lt;stdint.h&gt;
#include &lt;stdio.h&gt;

extern "C" {
    int32_t __stdcall HelloWorld();
}

int main() {
    printf("%d\n", HelloWorld());
}
</code></pre>
<p>You can build it with MSVC as follows:</p>
<pre><code>cl hello_world.cpp hello_world.dll.lib
</code></pre>
<p>The dumpbin tool can be used to further inspect imports and exports.</p>
<pre><code>&gt; dumpbin /nologo /exports hello_world.dll

Dump of file hello_world.dll

File Type: DLL

  Section contains the following exports for hello_world.dll

    00000000 characteristics
    FFFFFFFF time date stamp
        0.00 version
           1 ordinal base
           1 number of functions
           1 number of names

    ordinal hint RVA      name

          1    0 00001000 HelloWorld = HelloWorld
</code></pre>
<pre><code>&gt; dumpbin /nologo /imports hello_world.exe

Dump of file hello_world.exe

File Type: EXECUTABLE IMAGE

  Section contains the following imports:

    hello_world.dll
             140017258 Import Address Table
             140021200 Import Name Table
                     0 time date stamp
                     0 Index of first forwarder reference

                           0 HelloWorld
</code></pre>

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